Saturday, January 20, 2018

There's Never A Shortage of Things to Repair or Replace

I don't know; maybe for some of you.  For me, there's a perpetually self-filling list of things to look after.

For the uninitiated, there's a very common and exceedingly useful piece of electronics test equipment called an oscilloscope, generally just called a scope.  In quick overview, an oscilloscope allows you to observe voltage waveforms; that is, the voltage vs. time in a circuit that's under test.  There are volumes written about them.

I have one which isn't working.  I bought a surplussed-out scope from my employer about 10 years ago, a Tektronix 475, which is an advanced model for a hobbyist, but primitive compared to modern scopes.  Like a lot of equipment nearing its end of life, these are prone to failures and mine did.  Last year, while working on my CNC control box, I had some signals I wanted to look at so I rolled the scope over to the shop.  When I turned on the oscilloscope the display wasn't acting right, and after a few seconds I got the unmistakable whiff of smoke coming out of it.  Since I was busy, I put it aside to troubleshoot later.  Being concerned about the age and safety of applying power to the scope is one of the reasons I built the variac box

This week was time to open it up and see if I could find the trouble.  A 475 is one of a series of several models that Tektronix introduced beginning in the mid-60s, where higher numbers indicate higher speed (from 453 to the 485).  Mine has parts inside with what seem to be 1978 date codes, so it's either from late '78 or early '79.  They're also sometimes described as "15 pounds of parts in a 5 pound bag".  
To get more to the story, between the two of us, Mrs. Graybeard and I have about 70 years experience as electronics technicians (being a good technician is often one of the most important parts of being an engineer).  For the last 30 years, I mostly worked on debugging things that had never worked, while she primarily was fixing things that once had.  The two of us dug into the scope as a team.  Much to my surprise, when I pulled the cover, there was no burned spot where something had obviously smoked.  She quickly went into full detective mode, sniffing the thing trying to localize the strongest scent of burned parts and sort of pushing me out of her way.  Kind of surprisingly, she fairly quickly found two high voltage diodes that were open - as in blown out.  Good diodes measure low resistance in one direction and higher resistance in the other; these both appeared very high resistance in both directions.  We spent much of the day trying to figure out what could have blown them up and don't really have an answer.  Diodes do "just fail", but two at once is way too much coincidence.

These are much higher voltage diodes than anything I have: they're rated 5000 Volts.  My searches for replacements were (1) nobody's got them, then (2) I have to buy a reel of parts for something like $56,000, and finally (3) I found that Mouser Electronics had an equivalent I could buy 10 for 76 cents each with no minimum order.  Doesn't $7.60 sound so much better than $56,000?  I got 10 so that we could replace the diodes and turn the thing on.  It's possible that will fix it, and it's possible these will blow up as well.  I'm betting it's more likely we blow up a couple more diodes.  But then maybe we can isolate what's going on better.  We have five chances. 

Fixing electronics is my "home field advantage", and while the scope is rather inconvenient to work on, it's all parts with leads and that's easier to work on than a lot of modern electronics.  But it's still a PITA. 

Adaptive Curmudgeon just posted a very relevant piece that sometimes it's really overwhelmingly tempting to say "eff this" and if I could find a guy who's expert in fixing these things that would do so reasonably (a whole replacement scope is probably $300) I probably would have someone else fix it.  If I can fix it for the $7.60 in diodes and some parts from my junk collection, it's worth it.


  1. I've got a 2465B, and it's the nicest scope I've ever owned!

  2. Fix the tools to fix the tools to fix the tools........


  3. And you bought five chances for less than ten bucks
    Now if I could figure out how to fix my pocket radios.

  4. I SWAG it could be a shorted cap in the CRT area.
    There, pinned it right down for ya.

  5. If by chance you find the old CRT model won't come back to life, have a look at the Rigol o'scopes: they have a good rep in the hobby world, and if you want a really capable portable logic analyzer/scope the Saleae USB one is really hard to beat. Both can be had for under $400, and for non-professional work they're more than good enough. But trying the under $10 option first is wise, certainly!

    1. They're on my short list. A friend bought one a few years ago and it was impressive. This is a 200 MHz oscilloscope, and it looks like a dual channel Rigol is over $800 while to replace this with another one is more like $250, so there is that.

      All modern instruments are computer with the necessary analog hardware attached, and the LCD screen gets rid of the HV supply, which is my problem right now. The other side of getting another 475 is that it's cheaper, but is it going to last as long a newer scope?

  6. After two different repair shops gave up, an old mechanical Pfaff sewing machine is sitting on the dining room table in pieces. That is a complex piece of kit- a collection of shafts, cams, gears, pitman arms ,eccentrics to baffle Rube Goldberg. And every one adjustable, so finding a fixed starting position for adjustment is like trying to hold smoke. I have wondered if the piece is unnecessarily complex, like so many German designs. And then I started wondering what the maintenance hours per hour of flight time were on a P48 or P51 vs a FW190 or a BF109......
    So my wife and I wander in from time to time and work on it, trying to delve out it's secret handshake to get it working again...

    1. I spent a week on my wife's Singer going through it and replacing anything that looked worn, and flushing out old lube, replacing it with some synthetic stuff.

      It will probably now outlast us both.....

  7. I still have my (now old) 485 in working condition (quick prayers). Even has the camera mounts as well (no camera though).

    The new scopes may be better in many ways, but I still have a fondness for the old analog scopes where I twist a knob instead of searching through a menu.

    A whole lot of today's technology is built on a foundation of yesterday's technology. Meaning yesterday's technology is still in use. A scan through DigiKey indicates a lot of 70s and 80s ICs are still top sellers. And in packages I can see and still solder by hand.

    Good luck with the repairs - it's a good scope.


  8. I haven't fired up my scope in awhile, it's not as nice as a Tek 475 either. I was motivated to fix the central vac system after it failed right before Thanksgiving during the final house cleanup. We tried a repair place who blew us off when they found out how far away we were, and we had to use the portable vacuum (not a good substitute). I was however able to diagnose the failed control board, find a $43 universal substitute board on line instead of the $200+ official replacement part and get it going again. Now I know a lot about central vac systems, and I really impressed my wife.